1. Field of the Invention
The present invention relates to a wafer counter device serving to count the number of wafers in a cassette and align the orientation flats (which will be hereinafter referred to as O.F. or orifla) of the wafers.
2. Description of the Related Art
Wafer counter is used at various lots in the course of manufacturing semiconductor devices so as to confirm the presence of the semiconductor wafers in a cassette and the number of the wafers in the cassette, particularly when batch heat-treating of plural wafers, which will be supplied to a device at a next process. In short, the wafer counter serve to detect the number of the wafers in cassette. The wafer counter of this type is disclosed in the Japanese Utility Model Disclosure Sho 61-129340.
On the other hand, the Japanese Patent Publication Hei 1-59739 discloses a technique for aligning the oriflas of plural semiconductor wafers in a cassette with one another by rotating rollers. According to this technique, however, each of the wafers in grooves of the cassette is contacted with the inner wall at upper and lower portions of the groove when it is rotated by the rotating rollers. This causes the wafers to be cracked at their contact surfaces and film bonded to them according to the CVD process to be peeled off from them. Namely, a large amount of the so-called chippings is thus caused. As the result, particles created by the chippings float in the air and adhere to the wafers, thereby decreasing the productivity of the semiconductor devices.
The Japanese Patent Disclosures Sho 63-13344 and 64-743 disclose a technique for detecting the presence of plural wafers in a cassette at once, using the same number of optical position detectors as that of the wafers per cassette.
When the number of the wafers in a cassette is to be counted after the oriflas aligning process according to the techniques disclosed in the above-cited references, each of the wafers rotated by the rotating rollers which serve to align the oriflas of the wafers with one another is moved range of 2 mm, maximum, from the center of that groove of the cassette, in which the wafer is seated, in direction of the thickness of the wafer.
The pitch of grooves on the inner wall of the cassette which is usually used is 3/16 inches (4.76 mm) and the interval between the two adjacent wafers in the cassette is represented by this pitch of grooves. If the optical position detector can be made as thin as possible to be inserted into the interval between the two adjacent wafers in the cassette and the possible thickness of it is 2 mm, for example, the distance between the wafer and the optical position detector inserted is only about 1 mm.
When each of the optical position detectors is moved under this state to insert it between the two adjacent wafers in the cassette, the wafers which may be greatly shifted from the centers of their grooves of the cassette are contacted with the optical position detector at their peripheral portions. As the result, a large amount of the chippings is created from the wafers thus contacted, thereby decreasing the productivity of semiconductor devices.
Further, the oriflas of the wafers in a cassette are aligned with one another by the aligning means and the wafers in the cassette are then counted by the wafer counter at a different process in the conventional cases. Therefore, it takes a long time to process a lot of the wafers. This is the reason why it is desired to further enhance the throughput.